Microcystins

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Microcystins are toxins produced by cyanobacteria. Cyanobacteria are also known as blue-green algae and are ubiquitous in surface water when conditions are favorable for growth and formation of algal blooms. Cyanobacteria release toxins upon cell death or lysis. When released, toxins may persist for weeks to months. [564] Toxins of most concern are microcystins. Microcystins take their name from the genera Microcystis. Most microcystins are hepatotoxins (liver toxins). [564] Hepatotoxins are produced by species of the genera Microcystis, Anabena, Nodularia, Oscillatoria among others. [567] Most microcystins are associated with Microcystis aeruginosa. While the liver is the primary target of microcystins, it is also a skin, eye and throat irritant. [567]

Microcystins are cyclic heptapeptides with seven amino acids. Microcystins are named for the various amino acids on the peptide structure. Microcystin-LR is named for the leucine (L) and argentine (R) amino acids. Microcystin-LR was the first identified and is the most commonly studied. Other common microcystins are RR, YR and LA. [565]

Cyanobacterial blooms can persist with adequate levels of phosphorous and nitrogen, temperatures in the 5 to 30 C range and pH in the 6 to 9 range, with most blooms occurring in late summer and early fall. [567] Microcystin toxins are nonvolatile, hydrophilic, and stable in sunlight, and stable over a wide temperature and pH range. [565] Factors affecting toxin production are light and temperature, with optimum temperatures from cyanobacteria ranging 20 to 25 C. [567] These conditions suggest microcystins present in surface water supplies in warm and sunny climates.

Microcystins are not regulated by USEPA in drinking water, but are unregulated microbial drinking water contaminants listed on the USEPA's Contaminant Candidate Lists (CCLs) 1 and 2 as cyanobacteria and their toxins. [564] Microcystins, specifically Anatoxin-A, Microcystin-LR and Cylindrospermopsin, are on the CCL3. [620] The WHO has established a provisional guideline provisional of 1 ug/L for microcystin-LR. [567]

Reference 564 cites a report of the AWWA Research Foundation wherein 80 percent of samples in Canada and US were positive for microcystins and 4.3 percent were above the WHO guideline of 1 ug/L. Microcystins were reported in raw waters for two Alberta supplies ranging 0.15 to 4.3 ug/L with wide fluctuation over time. [568] A study of 160 surface water supplies in Manitoba reported microcystin-LR detected in 70 percent of raw water supplies ranging up to 1.0 ug/L. [569] Microcystin-LR was detected over a June to December period in Manitoba water supplies. It was found to persist for up to two months following the decline of algal populations. [570]